PHENYLENE RING DYNAMICS IN 1,4-DIPHENOXYBENZENE

The crystal structure for 1,4-diphenoxybenzene has been determined; M = 262.3, space group P21/c, a = 5.856(2), b = 7.822(2), c = 15.422(4) Å, β = 95.24(2), U = 703.7 Å, Z = 2. Molecules of 1,4-diphenoxybenzene adopt a structure with trans coplanar terminal rings and the central benzene ring is orthogonal to these giving torsion angles close to 90°. Chemical shift inequivalences are seen in the 13C CPMAS NMR spectra which can be attributed to an electric-field effect from the electric dipole of the ether group. Using these chemically inequivalent resonances the phenylene ring dynamics have been investigated. The terminal phenyl rings were found to undergo π 'flips' with an activation energy of 59.1 kJ mol -1. Indirect estimates of the 'flip rate' for the central ring show that it is flips much more slowly than the terminal rings.